The present invention relates to a projector which projects an image on a screen using reflection liquid crystal panels such as a reflection liquid crystal projector, a projection type image display apparatus such as a reflection liquid crystal rear projector, and a projection type image display apparatus optical unit.
An optical unit for a reflection liquid crystal projector which uses reflection liquid crystal panels is explained next. Non-polarized light coming from a light source is linearly polarized by a polarization converter and cast on a polarizer. The unwanted polarized component is removed from the light by the polarizer before it reaches a reflection liquid crystal panel. The light is modulated by the reflection liquid crystal panel so that its polarization state is changed on a pixel-by-pixel basis according to image signal. Then, it reaches an analyzer. The quantity of light which is transmitted or reflected by the analyzer is determined according to the polarization state of the light emitted from each pixel. An image thus obtained is projected in an enlarged form by a projection lens. A polarizing beam splitter prism (hereinafter called a PBS prism) is commonly used as a polarizer/analyzer. A PBS prism has a dielectric multilayer film (hereinafter called a PBS film) which transmits p-polarized light and reflects s-polarized light.
In a reflection type liquid crystal projector which uses a PBS prism as a polarizer/analyzer, light leakage from the PBS prism occurs for the following reason when a black image is displayed. The s- and p-polarization directions of light rays which are not parallel to the main incidence (entrance) plane are different between when they are transmitted or reflected by the PBS prism and when they reenter the PBS prism after being reflected by the reflection liquid crystal panel. However, after being transmitted or reflected by the PBS prism, rays retain their polarization direction when they reenter the prism. Therefore, rays parallel to the main incidence plane are completely transmitted or reflected when they reenter the prism; on the other hand, for rays which are not parallel to the main incidence plane, the s-polarized component is reflected and the p-polarized component is transmitted. For this reason, rays which are not parallel to the main incidence plane may cause light leakage, resulting in black image contrast deterioration.
Also, a contrast improvement method which uses a quarter-wave plate between a projection lens and a cross dichroic prism has been suggested.
However, in the case that a PBS prism is used as a polarizer/analyzer for a reflection type liquid crystal panel as mentioned above, even when a quarter-wave plate is disposed just before the panel in order to prevent contrast deterioration, the effect of the quarter-wave plate is not satisfactory. A wave plate has specific wavelength and angle characteristics. As the difference of the incident light wavelength from the design center wavelength increases or the incidence angle increases, the performance of the wave plate decreases, so light leakage cannot be prevented completely and contrast deterioration will occur. Even when a polarizing plate is disposed between the PBS prism and the projection lens for the purpose of preventing leak light from being projected on the screen, it is impossible to prevent light leakage completely because the leak light includes rays polarized in the same direction as the direction of the polarizing plate transmission axis.
Besides, the use of the PBS prism is disadvantageous from the viewpoint of weight. Furthermore, when the PBS prism uses a glass material with a low photoelastic coefficient to avoid birefringence which might cause contrast deterioration, it may be relatively heavy because of the relatively large specific gravity of the glass material, and costly because the material is not widely available on the market.